1. Field of the Invention
The present invention relates to an image printing apparatus and, more particularly, to a technique of printing an image in an image printing apparatus by transferring a toner image formed on a photosensitive drum onto a transfer sheet and thermally fixing the image.
2. Description of Related Art
Electrophotographic image printing apparatuses have been conventionally used for copying machines, printers, and the like. In an electrophotographic image printing apparatus, first of all, while a uniformly charged photosensitive drum as an image carrying body is moved (rotated), its surface is exposed to a laser beam to form an electrostatic latent image on the photosensitive drum, and the formed electrostatic latent image is developed to form a toner image on the photosensitive drum. A transfer separation portion then transfers the formed toner image onto a transfer sheet and separates the transfer sheet from the photosensitive drum. The transfer sheet onto which the toner image is transferred is sandwiched and conveyed by the heat roller and pressure roller of a fixing section, and the image is fixed with heat and pressure, thereby performing image printing operation.
In such an image printing apparatus, toner is not fixed in the interval between the transfer separation portion and the fixing section. If, therefore, a transfer sheet is sandwiched and conveyed by the roller pair, the toner image on the transfer sheet is disturbed, and the roller pair is stained. For this reason, the transfer sheet is conveyed while being placed on, for example, a conveyor belt. The transfer sheet cannot therefore be conveyed accurately as compared with the case wherein the roller pair is used. In addition, when a transfer sheet enters between the heat roller and pressure roller of the fixing section, the sheet may strike one of the roller surfaces and slip to cause a delay in conveyance. Owing to such problems in conveyance, if the interval between transfer sheets which are continuously conveyed (i.e., the interval between the trailing edge of a transfer sheet which is transmitted first and the leading edge of a transfer sheet which is conveyed next; to be referred to as an inter-sheet distance hereafter) is not sufficient, the transfer sheets may overlap. It therefore cannot help but ensure a sufficient inter-sheet distance between transfer sheets. Ensuring a sufficient inter-sheet distance will decrease the image printing efficiency.
In an image printing apparatus including a transfer sheet reversing section which reverses the convey direction of a transfer sheet and reverses the obverse and reverse surfaces of the transfer sheet so as to perform reverse delivery and duplex image printing, it is necessary to ensure a large inter-sheet distance to prevent the next transfer sheet from entering the transfer sheet reversing section during sheet reversing operation. Since the image printing efficiency decreases as the inter-sheet distance increases, some apparatus is designed to prevent a decrease in image printing efficiency by increasing the convey velocity of transfer sheets. Even this technique cannot prevent the above trouble in conveyance. In addition, since slip may occur between, for example, convey rollers and a transfer sheet when the convey direction is reversed, if a sufficient inter-sheet distance cannot be ensured between transfer sheets to be continuously conveyed, the transfer sheets may collide with each other. It is therefore necessary to ensure a sufficient inter-sheet distance in consideration of these troubles. This leads to a decrease in image printing efficiency.
In order to improve the image printing efficiency, the image printing speed may be increased. An increase in image printing speed is limited by deterioration in image quality, sticking of a transfer sheet to the photosensitive drum, and a restriction on the rotational speed of a polygon motor used for laser beam scanning. Under the circumstances, an improvement in image printing efficiency may be achieved by minimizing the inter-sheet distance. For example, there is provided an image printing apparatus which determines whether the distance to a preceding transfer sheet is a predetermined distance or more, and ensures a sufficient inter-sheet distance by temporarily decreasing the convey velocity of the transfer sheet if the distance is less than the predetermined distance (see Japanese Unexamined Patent Publication No. 2001-341898 (patent reference 1)). However, the image printing apparatus disclosed in patent reference 1 is designed to correct the problem of an inter-sheet distance shortage which occurs at the time of feed operation, but is not designed to correct variations in inter-sheet distance due to problems in the above image printing process and problems in conveyance at the time of sheet reversing operation. If, for example, the convey velocity is changed during the printing process to ensure a sufficient inter-sheet distance, a deterioration in image quality occurs.
Recently, many post-processing apparatuses for electrophotographic image printing apparatuses have been contrived to perform bookbinding post-processing for delivered transfer sheets, e.g., collation, stapling, and center folding/saddle stitching. As the above variations in inter-sheet distance occur, the timing of the delivery of transfer sheets, i.e., the timing at which the post-processing apparatus receives a transfer sheet, varies. In some cases, mechanical operation in the post-processing apparatus cannot cope with large variations in the timing of the delivery of transfer sheets.
Demands therefore have arisen for a technique of correcting variations in inter-sheet distance during conveyance, improving image printing efficiency by minimizing the inter-sheet distance, and stabilizing the timing of delivery to allow a post-processing apparatus to cope with such variations.